Does system energy "superpose"? That is, if you have one system of charges with total stored energy $W_1$, and a second charge distribution with $W_2$...if you superpose these charge distributions, is the total energy of the new system simply $W_1 + W_2$? 1. Yes 2. No Note: * CORRECT ANSWER: B * Draw 4 charges and show that it is not the sum of the 2 charges and the other 2.

## Announcements * Homework 5 has a partner problem - Review problem that you share with each other - Can share on Piazza (for extra credit!) * Exam 1 is Wednesday (7-9pm in A149 PSS)

<img src="./images/pt_charges_energy.png" align="center" style="width: 300px";/> Two charges, $+q$ and $-q$, are a distance $r$ apart. As the charges are slowly moved together, the total field energy $$\dfrac{\varepsilon_0}{2}\int E^2 d\tau$$ 1. increases 2. decreases 3. remains constant Note: * CORRECT ANSWER: B * Consider when they overlap, field goes to zero, must be E gets smaller as they get closer. same volume

<img src="./images/capacitor_pull_apart.png" align="center" style="width: 500px";/> A parallel-plate capacitor has $+Q$ on one plate, $-Q$ on the other. The plates are isolated so the charge $Q$ cannot change. As the plates are pulled apart, the total electrostatic energy stored in the capacitor: 1. increases 2. decreases 3. remains constant. Note: * CORRECT ANSWER: A * Same E; constant; larger volume where it is non-zero

### Conductors <img src="./images/electron_sea.gif" align="center" style="width: 700px";/>

### The conductor problem <img src="./images/metal.png" align="center" style="width: 500px";/>